Climate of Idaho

Topographic Features

Idaho lies entirely west of the Continental Divide, which forms its
boundary for some distance westward from Yellowstone National Park.
With a maximum north-south extent of 7° of latitude, its east-west
extent of 6° of longitude at latitude 42° N., but only 1°
of longitude at 49° N. The northern part of the state averages
lower in elevation than the much larger central and southern portions,
where numeous mountain ranges form barriers to the free flow of air
from all points of the compass. In the north the main barrier is the
rugged chain of Bitterroot Mountains forming much of the boundary
between Idaho and Montana. The extreme range of elevation in the
state is from 738 feet of the confluence of the Clearwater and Snake
Rivers to 12,655 feet at Mt. Borah in Custer County. Comprising
rugged mountain ranges, canyons, high grassy valleys, arid plains,
and fertile lowlands, the state reflects in its topography and
vegetation a wide range of climates. Located some 300 miles from the
Pacific Ocean, Idaho is, nevertheless, influenced by maritime air
borne eastward on the prevailing westerly winds. Particularly in
winter, the maritime influence is noticeable in the greater average
cloudiness, greater frequency of precipitation, and mean temperatures,
which are above those at the same latitude and altitude in
midcontinent. This maritime influence is most marked in the northern
part of the state, where the air arrives via the Columbia River Gorge
with a greater burden of moisture than at lower latitudes. Eastern
Idaho's climate has a more continental character than the west and
north, a fact quite evident not only in the somewhat greater range
between winter and summer temperatures, but also in the reversal of
the wet winter-dry summer pattern.

Temperature

The pattern of average annual temperatures for the state indicates the
effect both of latitude and altitude. The highest annual averages are
found in the lower elevations of the Clearwater and Little Salmon
River Basins, and in the stretch of the Snake River Valley from the
vicinity of Bliss downstream to Lewiston, including the open valleys
of the Boise, Payette, and Weiser Rivers. At Swan Falls the annual
mean is 55° F, highest in the state. Obsidian, at an elevation
of 6,780 feet in Custer County, has the lowest annual average,
35.4° F, of any reporting station, with such places as Sun
Valley, Chilly Barton Flat, Grouse, Island Park Dam, and Big Creek
not far behind. The range between the mean temperature of the coldest
and warmest months of the year varies from less than 40° F at a
number of northern stations, to well over 50° F at stations in
the higher elevation of the central and eastern parts of the state.
In the basin of the Snake River and its tributaries, between Twin
Falls and Idaho Falls, monthly mean temperatures of 32° F or
lower persist from December through February, while downstream from
Twin Falls, at the lower elevations, monthly mean temperatures are
freezing or below only in December and January. Low-level stations
like Riggins and Lewiston show no month in the year with mean
temperature 32° F or lower. In general, it can be said that
monthly means are 32° F or lower at stations above 5,000 feet from
November through March; between 4,000 and 5,000 feet, November through
February; 3,000 to 4,000 feet, December through February; and 2,000
to 3,000 feet, only one or two months. The diurnal range of
temperature is, of course, most extreme in high valleys and in the
semiarid plains of the Snake River Valley. The magnitude of diurnal
range varies with the season, being lowest in winter when cloudiness
is much more prevalent, and greatest in the warmer part of the year.
At Boise, for example, the average diurnal range is only 14° F in
January, but exceeds 30° F in July through September. Temperatures
can range from -60° to 118° F. The coldest monthly mean
minimum temperature has been -20° F, and the warmest monthly mean
maximum 104° F. The highest long-term annual average has been
55°F at Swan Falls Power House, and the lowest long-term average
35° F at Obsidian. In summer, periods of extreme heat extending
beyond a week are quite rare, and the same can be said of periods of
extremely low temperatures in winter. In both cases the normal
progress of weather systems across the state usually results in a
change at rather frequent intervals. In the realm of extremely low
temperatures, two winters stand out in the records for the state:
1937-38 and 1948-49. The lowest monthly mean temperatures on record
occurred throughout the state in January 1949, and many stations
registered the absolute lowest temperature on record during that
month.

Precipitation

To a large extent the source of moisture for precipitation in Idaho is
the Pacific Ocean. In summer there are some exceptions to this when
moisture-laden air is brought in from the south at high levels to
produce thunderstorm activity, particularly in the eastern part of
Idaho. The source of this moisture from the south is apparently the
Gulf of Mexico and Caribbean region. The average precipitation map
for Idaho is as complex as the physiography of the state. Partly
because of the greater moisture supply in the west winds over the
northern part of the state, (less formidable barriers to the west)
and partly because of the greater frequency of cyclonic activity in
the north, the average valley precipitation is considerably greater
than in southern sections. Peaks on the average annual precipitation
map are found, however, in nearly all parts of the state at higher
elevations. Sizeable areas in the Clearwater, Payette, and Boise
River Basins receive an average of 40 to 50 inches per year, with a
few points or small areas receiving in excess of 60 inches. Large
areas including the northeastern valleys, much of the Upper Snake
River Plains, Central Plains, and the lower elevations of the
Southwestern Valleys receive less than 10 inches annually. Seasonal
distribution of precipitation shows a very marked pattern of winter
maximum and midsummer minimum in the northern and western portions of
the state. In the eastern part of the state, however, many reporting
stations show maximum monthly amounts in summer and minimum amounts
in winter. In the Northeastern Valleys and Eastern Highlands, more
than 50 percent of the annual rainfall occurs during the period April
through September. Over nearly all of the northern part of the state,
however, less than 40 percent of the annual rainfall occurs in this
same period, and in portions of the Boise, Payette, and Weiser River
drainages less than 30 percent of the annual amount comes in that
six-month period.

Snowfall

Snowfall distribution is affected both by availability of moisture and
by elevation. Annual snowfall totals in Shoshone County have reached
nearly 500 inches. The greatest long-term (1942-56) seasonal average
was 182 inches at Mullan Pass, while the greatest snow depth (also
182 inches) was recorded at that station on February 20, 1954. The
major mountain ranges of the state accumulate a deep snow cover
during the winter months, and the release of water from the melting
snowpack in late spring furnishes irrigation water for more than two
million acres, mainly within the Snake River Basin above Weiser.
Irrigation water supplies are nearly always plentiful, except on some
of the smaller projects where storage facilities are inadequate.
Electric power in increasing amounts is generated by the waters of the
many rivers of the state.

Floods

Floods in Idaho occur most often during the period of seasonal
snowmelt in spring, particularly in April and May. A few areas in the
state are actually flooded or threatened by flood waters nearly every
year. The Kootenai River Valley, in the vicinity of Bonners Ferry, is
one such area, and another is the Snake River upstream from Idaho
Falls in the vicinity of Roberts and Menan. Much has been done to
minimize the damage from such seasonal floods through the construction
of reservoirs and other flood-control facilities. So-called "out of
season" floods do occur occasionally at a number of points in the
state. For example, wintertime floods occurred on the Payette, Weiser,
Little Salmon, and Wood Rivers and portions of the upper Boise and
Payette Basins in 1955, 1957, and 1958. Flash floods on small streams,
or occasionally in ravines or dry gulches, occur a few times each
year as the result of heavy rains associated with thunderstorms. Two
areas that seem to be particularly susceptible to this type of
flooding lie between Downey and Pocatello in the southeast, and in the
north-central part of the state between Grangeville and Moscow.

Humidity

Humidity, as a climatic factor, seems to hold a great deal of interest
for many people, yet it is one of the more elusive weather elements
to deal with statistically. The diurnal range of relative humidity
generally follows a pattern which is the reverse of the diurnal
temperature curve. Precipitation or fog interferes with such a
pattern, but the averages show maximum humidity at the time of minimum
temperature and vice versa. In winter, average relative humidities
are considerably higher than during the hot weather. Human comfort
during the summer months is greatly affected by the moisture content
of the air. In Idaho, where maximum temperatures above 90° are
not uncommon in July and August, humidity at the time of maximum
temperature is usually below 25 percent, and often down to 15 percent
or lower. With any kind of air movement the higher temperatures are
quite within the range of adjustment of the human system. The low
relative humidity of summer also permits widespread use of
evaporation-type coolers. Field drying of hay is facilitated by the
dry air, especially when air movement is sufficient. Some work has
been done on the drying of corn and grain without the addition of
heat, but in the season when this activity is carried on, the days
are shorter and the periods of lower relative humidity are rather
brief.

Fog

Fogs in Idaho are extremely variable, and statistics are available for
only a few places, mainly the cities where regular National Weather
Service Offices are maintained at airports — Boise, Lewiston,
and Pocatello. At Boise, heavy fog (visibility ¼ mile or
less) is experienced on an average of 17 days per year, with a
maximum of six occurrences in December. The year-to-year variation is
considerable, however, and twice in the early 1950's, December
experienced 15 days with heavy fog. Lewiston also averages 17 days
with heavy fog per year, with a peak of four each in November,
December, and January. Pocatello has an average of only 10 days of
heavy fog per year, the maximum monthly average being four in
January. Along with fogs, particularly if they persist for a few
days, rime ice occasionally forms on power and communication lines.
These occurrences are relatively infrequent, so much so that no real
statistics on frequency or critical areas are available.

Storms

Windstorms are not uncommon in Idaho, but the state has no destructive
storms such as hurricanes, and an extremely small incidence of
tornadoes. Windstorms associated with cyclonic systems, and their
cold fronts, do some damage to trees each year, often causing
temporary disruption of power and communication facilities, but only
minor damage to structures in most instances. Storms of this type may
occur at any time from October into July, while during the summer
months strong winds almost invariably come with thunderstorms. Hail
damage in Idaho is very small in comparison with damage in areas of
the central part of the United States. Often the hail that occurs
does not grow to a size larger than one-half inch in diameter, and
the areas affected are usually small. Quite often hail comes during
early spring storms, when it is mostly of the small, soft variety
with a limited damaging effect. Later when crops are more mature and
more susceptible to serious damage, hail occurs in widely scattered
spots in connection with summer thunderstorms. The incidence of
summer thunderstorms is greatest in mountainous areas, where
lightning often causes serious forest and range fires.

Sunshine

The annual average percentage of possible sunshine ranges from about
50 in the north to about 70 in the south. Winter, with its frequent
periods of cloudy weather, has about 40 percent of possible sunshine
in the large open valleys of the south and less than 30 percent in
the north. In July and August the average percentage rises to the
upper 80's in the southwest and to near 80 in the east and north.

Growing Season

The growing season (freeze-free period), like the average temperature,
varies greatly throughout the state because of differences in
elevations, soil type, topography, and vegetative cover. Year-to-year
variations are also considerable in many areas. The area in the
immediate vicinity of Lewiston has the longest growing season,
approximately 200 days, followed by a sizeable area in the central
Snake, and lower Boise, Payette, and Weiser River Basins with 150
days or more. Upstream along the Snake, the growing season shortens,
but is still of the order of 125 days in the Pocatello-Idaho Falls
area. A few high valleys have no month without freezing temperatures
and the land is used entirely for grazing.

Irrigated Lands

The upper portion of the Snake River Basin, upstream from the vicinity
of Bliss, has large areas of both irrigated and non-irrigated
farmland. At some of the higher elevations in the eastern part of the
state, the land is used primarily for livestock ranches and wheat
farms. At somewhat lower elevations, one of the principal crops is
the Idaho Russet potato, for which the upper Snake Basin is famous.
Approximately 90 percent of Idaho's potatoes are grown on about
110,000 acres of irrigated land on the Snake River plain. Another
100,000 acres, in the area between Gooding and Minidoka counties, are
devoted to the production of dry, edible beans. More than 50,000
acres are used for growing sugar beets, which rank high in importance
because of the large per-acre gross income. Hay, consisting primarily
of alfalfa, occupies more than one-third of the crop area in this
part of the state. Within this large region only about 9,000 farmers
reported dairy products as the chief source of income. From the
vicinity of Bliss on down to Weiser, the Idaho portion of the Snake
River Basin contains nearly three-quarters of a million acres of
irrigated land, which is a comparatively small percentage of the
total acreage of farms and stock ranches. The irrigated farms are
generally rather small, the majority being under 80 acres, but a
large variety of crops is grown. Sugar beets, onions, lettuce, hops,
mint, spring wheat, alfalfa, various seed crops, cherries, prunes,
peaches, and apples all contribute to the area's agricultural income.
Daily products also constitute an important part of the area's
output. The Clark Fork and Clearwater drainages of northern Idaho,
like the central Snake Basin in the southwest, have a relatively
small percentage of their total area in croplands. But the Camas and
Nez Perce praries, which form the eastern fringe of the famous
Palouse dry-farming area, are highly productive. A large part of
Idaho's winter wheat output comes from this area. Spring wheat,
barley, peas, and a few other crops are also grown, nearly all
without irrigation.

Forested Lands

Approximately two million acres of forest lie within the eastern part
of the state, mostly in the higher mountainous areas. Lumbering here
is done only on a small scale. The southwestern portion of the state
has a greater forested area, running into several million acres, and
lumbering is a more important phase of the economy than it is farther
east. The northern part of Idaho, because of its higher annual
precipitation, is more heavily forested than the southern portion of
the state, and lumbering has long occupied a prominent place in the
economy of the area. Lewiston, Potlatch, and Coeur d'Alene are among
the sites of important forest product industries. One of the finest
white pine stands in the country lies in northern Idaho, principally
in Clearwater and Shoshone Counties.

Recreation

Recreational resources are abundant in Idaho. The large lakes of the
north, particularly Coeur d'Alene, Pend Oreille, and Priest Lake,
provide wonderful boating and fishing, as well as serving as sites for
summer homes and camps. Payette Lake in Valley County and Bear Lake on
the Utah-Idaho border furnish recreational facilities for thousands of
persons each year. Henry's Lake in Fremont County furnishes some of
the best trout fishing in the state. Numerous reservoirs have added
greatly to the recreational facilities of the state, and hundreds of
mountain lakes and streams furnish almost unlimited opportunity for
camping, hiking, fishing, and hunting. Much of Idaho's area lies
within boundaries of several National forests, and the Forest Service
has set aside large tracts as wilderness areas. Within them, motorized
transportation, resorts, camps, homes, and logging are excluded.
Excellent hunting for deer, elk, bear, and Rocky Mountain sheep and
goats is afforted the hardier individuals who enter these more
inaccessible areas. The three principal areas are the Sawtooth
Wilderness, embracing a glaciated mountain range of that name which
forms the divide between the Salmon and Boise Rivers; the much larger
Idaho Wilderness, principally between the middle and south forks of
the Salmon River; and the Selway Wilderness area in the upper
Clearwater drainage.